The present invention relates generally to microwave antennas, and more particularly, to a microwave antenna having wide angle scanning capability.
Prior art teaches how a scanning line feed can be fed from its opposite ends to form two beams that are offset in space. Such scanning line feeds are described in articles by Nester, et al. entitled "Bidirectional Series Fed Slot Array", Symp. Digest, IEEE Antennas and Propagation Society International Symposium (Stanford, Calif.), June 1977, pg. 76, or Kinsey entitled "FAST Multibeam Antenna Concept", RADC-TR-85-170, Proc. Phased Arrays 1985 Symposium (Hanscom AFB, Mass.), September 1985, pp. 33-56.
There is increasing interest in low-cost, one-dimensional electronically scanned antennas for military and commercial applications, such as W-band targeting radars and forward looking radar automotive systems at 77 GHz, for example. Low-cost two-dimensional scanning can be realized by using the aforementioned one-dimensional electronic scan antenna in conjunction with a 360.degree. gimbal in the second axis. Conventional scanning techniques, such as PIN diodes, discrete ferrite phase shifters or transmit/receive (T/R) modules are generally either not available, not producible, or are relatively unaffordable at W-band.
Ferrite phase scanning antennas that radiate into free space are described in the technical literature. Ferrite phase scanning antennas are discussed in articles by Stern, et al. entitled "A mm-Wave Homogeneous Ferrite Phase Scan Antenna," Microwave Journal, Vol. 30, No. 4, April 1987, pp. 101-108, and U.S. Pat. No. 4,691,208 entitled "Ferrite Waveguide Scanning Antenna", for example.
A ferrite scanning line feed is a device that is similar to the previously discussed scanning line feed, and has the same limitations in scan range. The distinction is that the line feed is specifically designed to radiate into a parallel-plate region of a planar array antenna, which may be either air or dielectrically filled. Further, the scanning line feed is designed using bidirectional array excitation synthesis to be fed from either end, producing two well-formed beams offset in space.
A major disadvantage of the prior art is the limited scan coverage that can be realized with ferrite phase scanned antennas at high millimeter-wave frequencies, typically on the order of from 40 GHz and above. It would therefore be an advance in the art to have a planar array antenna that does not require a multitude of line feeds that are scanned with discrete phase shifters, such as those described in the Kinsey article, for example. It would also be an advantage to have a planar array antenna that has reduced complexity with much lower design and production costs. Accordingly, it is an objective of the present invention to provide for a microwave antenna having wide angle scanning capability.